Fluidbalanceconceptsinmedicine:Principlesandpractice

摘要： Theregulationofbodyfluidbalanceisakeyconcerninhealthanddiseaseandcomprisesthreeconcepts.Thefirstconceptpertainstotherelationshipbetweentotalbodywater(TBW)andtotaleffectivesoluteandisexpressedintermsofthetonicityofthebodyfluids.Disturbancesintonicityarethemainfactorresponsibleforchangesincellvolume,whichcancriticallyaffectbraincellfunctionandsurvival.Solutesdistributedalmostexclusivelyintheextracellularcompartment(mainlysodiumsalts)andintheintracellularcompartment(mainlypotassiumsalts)contributetotonicity,whilesolutesdistributedinTBWhavenoeffectontonicity.Thesecondbodyfluidbalanceconceptrelatestotheregulationandmeasurementofabnormalitiesofsodiumsaltbalanceandextracellularvolume.EstimationofextracellularvolumeismorecomplexanderrorpronethanmeasurementofTBW.Akeyfunctionofextracellularvolume,whichisdefinedastheeffectivearterialbloodvolume(EABV),istoensureadequateperfusionofcellsandorgans.Otherfactors,includingcardiacoutput,totalandregionalcapacityofbotharteriesandveins,Starlingforcesinthecapillaries,andgravityalsoaffecttheEABV.Collectively,thesefactorsinteractcloselywithextracellularvolumeandsomeofthemundergosubstantialchangesincertainacuteandchronicsevereillnesses.Theirchangesresultnotonlyinextracellularvolumeexpansion,butintheneedforalargerextracellularvolumecomparedwiththatofhealthyindividuals.Assessingextracellularvolumeinsevereillnessischallengingbecausetheestimatesofthisvolumebycommonlyusedmethodsarepronetolargeerrorsinmanyillnesses.Inaddition,theoptimalextracellularvolumemayvaryfromillnesstoillness,isonlypartiallybasedonvolumemeasurementsbytraditionalmethods,andhasnotbeendeterminedforeachillness.Furtherresearchisneededtodetermineoptimalextracellularvolumelevelsinseveralillnesses.Forthesereasons,extracellularvolumeinsevereillnessmeritsaseparatethirdconceptofbodyfluidbalance. ... （共28頁）